1. Field of the Invention
The present invention relates to radio signal transmitters, and more specifically, it relates to a radio signal transmitter employed in a radio base station formed by a master station and a slave station (forward station), for example, for receiving radio signals with two or more different frequency bands respectively, multiplexing the signals and transmitting the same between the master station and the slave station.
2. Description of the Background Art
In mobile communication through a portable telephone or a car phone, it is necessary to eliminate a blind zone such as underground or the inside of a tunnel where no radio waves from a radio base station reach. As that solving this problem, there is a radio base station formed by a master station without an antenna function and a plurality of slave stations having only the antenna function. The plurality of slave stations are dispersively arranged as forward stations in a blind zone or the like, and the master station and each slave station are connected to each other by an optical fiber, for example. In this case, signal transmission between the master station and the slave station is performed by an optical transmission system converting a radio signal (RF signal) to an optical signal and transmitting the same.
FIG. 13 is a block diagram showing an exemplary structure of a conventional optical transmission system employed in the aforementioned radio base station for optically transmitting a signal between the master station and the slave station. This type of optical transmission system is described in "Fiber-Optic Transmission System for Radio Base Stations" (Sanada et al., National Technical Report Vol. 39, No. 4, August 1993), for example.
Referring to FIG. 13, the conventional optical transmission system comprises an amplification part 90 amplifying an electric signal to be transmitted, an electrical/optical conversion part 91 performing electrical/optical conversion of an output signal from the amplification part 90, and an optical/electrical conversion part 92 performing optical/electrical conversion of the transmitted optical signal. The amplification part 90 and the electrical/optical conversion part 91 are provided on a sending end and the optical/electrical conversion part 92 is provided on a receiving end, while the electrical/optical conversion part 91 and the optical/electrical conversion part 92 are connected to each other by an optical fiber 93.
The electrical/optical conversion part 91 has, in relation to the power of the input signal, such a prescribed linear region that change of the strength of the output optical signal with respect to change thereof is linear. That is, when a signal having power exceeding the upper limit of this region is inputted in the electrical/optical conversion part 91, the output optical signal is distorted.
The optical/electrical conversion part 92 has, in relation to the strength of the input optical signal, such another prescribed linear region that change of the power of the output signal with respect to change thereof is linear. That is, when an optical signal having strength exceeding the upper limit of this region is inputted in the optical/electrical conversion part 92, the output signal is distorted.
The amplification part 90 has such an amplification factor that the power of the output signal from the optical/electrical conversion part 92 becomes sufficiently larger than that of noise while the power of the input signal in the electrical/optical conversion part 91 will not exceed the upper limit of the aforementioned prescribed linear region and the strength of the input optical signal in the optical/electrical conversion part 92 will not exceed the upper limit of the aforementioned another prescribed linear region. Thus, on the receiving end, a signal having sufficiently large power as compared with noise and with no distortion is obtained.
As described in "CDMA Cellular System" (Association of Radio Industries and Businesses, ARIB STD-T53 Version 1.0), in relation to mobile communication, as lines rapidly increases in number in recent years, there has been proposed employment of the CDMA (code division multiple access) system having a remarkably larger number of lines as compared with the conventional system. Recently mobile communication in the CDMA system is in part put into practice, and it is predicted that hereafter the ratio of the CDMA system occupying the mobile communication increases.
That is, the current system and the CDMA system coexist in the period up until shift to the CDMA system is completed and hence, in consideration of suppressing the facility cost, it is important to cope with the CDMA system while making the best use of the existing facility for the current system.
In the aforementioned conventional optical transmission system, consider optical transmission of an RF signal employed in the current system and a code division multiple access signal employed in the CDMA system. In this case, the receiving end cannot obtain, in relation to the code division multiple access signal, a signal having sufficiently large power as compared with noise. This is because, while in the current system and the CDMA system the power of signals is set in standards respectively, according to the standards, with reference to the power of the input signals supplied to the sending end, the power of the code division multiple access signal employed in the CDMA system is smaller than that of the RF signal employed in the conventional system.
The standards of the current system are described in "Digital Cellular Telecommunication System" (Research & Development Center for Radio Systems, RCR STD-27A) and the standards of the CDMA system are described in the aforementioned "CDMA Cellular System". An apparatus optically transmitting a code division multiple access signal is disclosed in Japanese Patent Laying-Open No. 6-70362 (Japanese Patent Application No. 4-219894), for example.
In relation to the code division multiple access signal, on the other hand, it is assumed that the amplification factor of the amplification part 90 is set high so that a signal having sufficiently large power as compared with noise is obtained. In this case, however, it is predicted in relation to the RF signal that the power of the input signal in the electrical/optical conversion part 91 exceeds the upper limit of the aforementioned prescribed linear region or the strength of the input optical signal in the optical/electrical conversion part 92 exceeds the upper limit of the aforementioned another prescribed linear region and consequently the signal obtained on the receiving end is distorted.
That is, when optically transmitting the RF signal and the code division multiple access signal in the aforementioned conventional optical transmission system, the receiving end cannot obtain, in relation to both of the RF signal and the code division multiple access signal, signals having sufficiently large power as compared with noise with no distortion whatever amplification factor of the amplification part 90 is set. Incidentally, the aforementioned Japanese Patent Laying-Open No. 6-70362 describes no means of optically transmitting both of the RF signal and the code division multiple access signal.
A problem similar to the above is quantitatively described from another point of view.
FIG. 14 is a block diagram showing the structure of a conventional radio signal transmitter.
Referring to FIG. 14, in the conventional radio signal transmitter a master station 200 and a slave station 300 are connected to each other by optical fibers 201 and 202. The optical fiber 201 is used when transmitting from the master station 200 to the slave station 300 (hereinafter referred to as a down system) an optical signal. The optical fiber 202 is used when transmitting from the slave station 300 to the master station 200 (hereinafter referred to as an up system) an optical signal.
The slave station 300 comprises an optical/electrical conversion part 301, an electrical/optical conversion part 303, a first amplification part 302, a second amplification part 304, a circulator 305 and an antenna 306.
First, down system signal transmission is described.
The optical signal sent from the master station 200 is transmitted through the optical fiber 201 to the slave station 300 present on a remote site. In the slave station 300, the optical/electrical conversion part 301 receives the optical signal sent from the master station 200 and converts the same to a radio modulation signal which is an electric signal. This radio modulation signal is amplified in the first amplification part 302 and thereafter radiated through the circulator 305 from the antenna 306. The circulator 305, which is an apparatus having a function of outputting an input from a certain terminal only to an adjacent terminal in a specific direction, outputs the input from the first amplification part 302 to the antenna 306 while outputting an input from the antenna 306 to the second amplification part 304 (as shown by arrows in FIG. 14). The radio modulation signal radiated from the antenna 306 is received by a mobile terminal (not shown) in the area.
Then, up system signal transmission is described.
Radio modulation signals having different frequencies sent from respective mobile terminals in the area are respectively received and frequency-multiplexed by the antenna 306. This frequency-multiplexed radio modulation signal is inputted through the circulator 305 into the second amplification part 304. The second amplification part 304 amplifies the inputted radio modulation signal and outputs the same to the electrical/optical conversion part 303. The electrical/optical conversion part 303 converts the radio modulation signal inputted from the second amplification part 304 to an optical signal and outputs the same. This optical signal converted in the electrical/optical conversion part 303 and thereafter outputted is transmitted through the optical fiber 202 to the master station 200 present on a remote site.
In the aforementioned structure of the slave station 300, the difference in the distances between the respective mobile terminals and the slave station 300 results in remarkable difference in received power received in the antenna 306. Therefore, the conventional radio signal transmitter employs in consideration of this difference in received power an extremely wide dynamic range for the signal of the up system and sets the optical modulation index per wave high.
An exemplary system design of this slave station 300 is described in the above-mentioned literature "Fiber-Optic Transmission System for Radio Base Stations" by Sanada et al. In this literature by Sanada et al., the optical modulation index m of the up system is, assuming that the up system has two carriers, 10.7%.ltoreq.m.ltoreq.21.2%. The lower limit and the upper limit of this optical modulation index m are decided respectively by carrier-to-noise ratio (CNR) characteristics and distortion characteristics.
FIG. 15 shows the relation between the optical modulation index, CNR and distortion (in this case, "distortion IM3" which is tertiary distortion).
As understood from FIG. 15, the CNR increases with increase of the optical modulation index, and the distortion IM3 is degraded by the increase of the optical modulation index. First, the optical modulation index at the lower limit is decided by a value satisfying CNR=80 dB, and the current value of the optical modulation index is 10.7%. On the other hand, the optical modulation index at the upper limit is decided by a value satisfying distortion IM3=-84 dBc, and the current optical modulation index is 21.2%. This distortion IM3=-84 dBc is, assuming that the distortion characteristic in the overall transmitter is -80 dBc, a distortion quantity which can be allowed by a semiconductor laser diode (LD) module employed as an optical/electrical conversion part in an optical sender.
While the aforementioned literature by Sanada et al. makes performance evaluation in the case of two carriers, in an actual system carriers for the up system are multi-carriers. In the literature by Sanada et al. up to 32 carriers at the maximum are assumed. In this regard, evaluation of distortion characteristics made with two carriers is made with multiple carriers.
In the case of multi-carrier transmission, not the distortion IM3 per wave but distortion of a composite triple beat (CTB) must be taken into consideration as the distortion characteristics. This distortion CTB in multi-carrier transmission is given by a composite number which is the number of tertiary distortion caused at the same frequency and the distortion IM3.
The relation between the aforementioned composite number and the carrier number is, as described in literature "Optical Feeder Basic System Design for Microcellular Mobile Radio" by Junji Namiki et al. (IEICE TRANS. COMMUN., VOL. E76-B, No. 9 September 1993, pp. 1069 to 1077), obtained by the following equation (1): EQU Nc=M*(N-M+1)/2+((N-3).sup.2 -5)-(1-(-1)).sup.-N *(-1).sup.N+M (1)
In the above equation (1), N represents the carrier number, M represents an M-th frequency band in N carriers, and Nc represents the composite number in the M-th frequency band. FIG. 16 shows a result obtained by calculating the relation between the carrier number and the composite number with the above equation (1).
As to the relation between the distortion IM3 and the distortion CTB in the same frequency band, assuming that D2 [dBc] represents the distortion IM3 caused when transmitting two carriers having a optical modulation index m2 [%] and DN [dBc] represents the distortion CTB caused when transmitting N carriers having a optical modulation index mN [%] of the same transmission system, DN is estimated with the composite number Nc in the following equation (2): EQU DN=D2+10*log(Nc)+2*20*log(mN/m2) (2)
Assuming that the value D2 of the distortion IM3 when mN=10.7% and 21.2% and m2=20% is -85 dBc, the relation between the distortion quantity DN and the composite number when mN=10.7% and 21.2% can be obtained from the above equation (2). FIG. 17 shows this relation. Obtaining from FIG. 17 a composite number satisfying -84 dBc which is the spec of the distortion characteristic DN of an LD module, it becomes "15" when mN=10.7% and becomes "1" when mN=21.2%. Further, the carrier number can be obtained from the relation between the composite number and the carrier number shown in FIG. 16, such that the carrier number becomes "8" carriers when the modulation factor is 10.7%, and becomes "3" carriers when the modulation factor is 21.2%.
The aforementioned carrier number is the number of mobile terminals performing sending in a certain area at a place closest to the slave station 300. In the range of this carrier number, influence exerted by tertiary distortion resulting from combination of the carriers on a signal sent at a place most separate from the slave station 300 (the receiving level in the antenna 306 is at the minimum) is in an allowable range.
However, the aforementioned conventional radio signal transmitter requires an extremely wide dynamic range for signals of the up system as described above, and allows the optical modulation index per wave to be set high. Therefore, when employing a communication system with an existing frequency band and a communication system utilizing another frequency band (e.g., a CDMA communication system) in the aforementioned conventional radio signal transmitter, such a problem arises that the distortion characteristic is degraded.
And this problem is essentially similar to the problem caused in the aforementioned conventional optical transmission system.
That is, when further performing CMDA communication in the aforementioned conventional signal transmitter, the CDMA signal (code division multiple access signal) is subjected to transmission power control and the received power in the antenna 306 is suppressed small and hence, in order to ensure sufficient CNR after optical transmission, the amplification factor of the amplification part 304 must be set large. However, the conventional radio modulation signal (RF signal) is not subjected to transmission power control and the received power in the antenna 306 may enlarge in some cases, and hence, assuming that the amplification factor of the amplification part 304 is set large as described above, there is a possibility that the conventional radio modulation signal is distorted.
Further, the aforementioned problem can arise not only in the case of optically transmitting the RF signal and the code division multiple access signal but also when transmitting a first radio signal and a second radio signal different in power from each other (this is not restricted to optical transmission either).